1. Field of the Invention
This invention relates generally to isomerization processes. More specifically this invention relates to the isomerization of light paraffins, in particular C.sub.6 paraffins.
2. Description of the Prior Art
Processes for the isomerization of paraffins into more highly branched paraffins are well known. New regulations affecting the composition of gasoline motor fuels have focused attention on isomerization to provide high octane pentane and hexane motor fuel components. A great deal of effort has been focused on maximizing the octane of isomerate streams by recovering and recycling the low-octane normal-paraffin isomers. In the more recent art, the octane of C.sub.6 isomerate streams has been additionally increased by also recovering and recycling the methyl-pentanes to obtain additional dimethylbutanes (DMB)'s. However, in all the prior art processes for hexane isomerization, 2,2-DMB is maximized despite the fact that 2,3-DMB has a higher octane. This is because these processes all operate close to thermodynamic equilibrium, where the concentration 2,3-DMB is much lower than that of 2,2-DMB.
U.S. Pat. Nos. 4,717,784 and 4,804,802 disclose processes for the isomerization of a hydrocarbon feed and the use of adsorptive separation to generate normal paraffin and monomethyl-branched paraffin recycle streams. The effluent from the isomerization zone enters a molecular sieve separation zone that contains a 5A-type sieve and a ferrierite-type sieve that adsorb normal paraffins and monomethyl-branched paraffins, respectively. U.S. Pat. No. 4,804,802 discloses stream or hydrogen as the desorbent for desorbing the normal paraffins and monomethyl-branched paraffins from the adsorption section and teaches that steam or hydrogen may be recycled with the normal paraffins or monomethyl-branched paraffins to the isomerization zone.